Production of conducting layers upon electrical resistors



Oct. 18, 1955 P. P. HOPF ETAL 2,721,153

PRODUCTION 'OF CONDUCTING LAYERS UPON ELECTRICAL RESISTORS Filed May 29,1950 Inventor P- 3 4 0, 25. J- L/M mud A itarney;

United States Patent 0 PRODUCTION OF CONDUCTING LAYERS UPON ELECTRICALRESISTORS Peter Paul Hopf, London, and Ronald Edgar John Lishmund,Reigate, England, assignors to Ward Blenkinsop & Company, Limited,London, England, a British com- P y Application May 29, 1950, Serial No.164,926

Claims priority, application Great Britain June 2, 1949 14 Claims. (Cl.117212) This invention relates to the production of printed electricalcomponents and printed circuits.

During the past few years an increasing demand for printed circuits hasarisen as their potentialities have been more fully appreciated and themethods of producing them have improved.

Several methods for producing printed circuits have been described. Oneis by painting the circuit on to the base material using a suitablypigmented paint: the painting may be done through a stencil. A variantthereof is to spray molten metal. Deposition of metal through a stencilas a result of simultaneously spraying separately prepared solutions ofsuitably reacting chemicals has also been used. Metal spattering throughstencils has been proposed.

In yet another type of process a foil of metal is bonded on to the basematerial and after applying a resist to those parts of the metal surfacein which a conductor is required the remaining metal is etched away andthe resist then removed. The usual bonding material employed is athermo-setting adhesive. A variant of this is a transfer process using apaper coated with pigment and resin, involving a hot stamping operation.

In operating any process of this character it is important to obtain auniform product with a minimum of rejects and in most of the knownprocesses it has been found that there are various difficulties whichlead to lack of uniformity and consequently the protection of rejects isfairly high.

We have now found a process for printing electrical components andprinted circuits upon organic electrical insulating materials in which ahigh degree of uniformity in the product is obtainable, which is simpleto control and in which unused conductor can be recovered for re-use. Inaddition overprinting can be readily carried out by this process.

According to the present invention there is provided a process forproducing a printed electrical element which comprises applying tothe'surface of an organic electrically insulating material a coatingsubstantially free from any permanent binder, of an at least lowelectrical conducting material in finely divided form, contacting thecoated material with a relief printing plate whilst simultaneouslyapplying heat andpressure, the temperature pressure and time employedbeing such as to cause the individual particles of conducting materialin the contact areas to cohere and to become at least partially embeddedin the underlying insulating material without substantially changing theelectrical properties of the surface thereof and thereafter removing thecoating of conducting material from those areas which have not been incontact with the printing plate.

The starting materials employed in the present process are a finelydivided form of the conductor such as finely divided silver or carbon.This is made up into a slurry in a volatile diluent to which may beadded a small proportion of a less volatile organic solvent which mayactas a temporary adhering agent. Examples of suitable diluents are thelower alkanols, especially methanol and ethanol, the cyclic ethers suchas dioxane, the lower aliphatic ketones, such as acetone and methylethyl ketone, and water. In selecting the diluent regard must be had forany undesirable action, such as a swelling action, which it might haveupon the surface of the insulating material. Typical examples ofconductors useful in the process are finely divided silver obtained byfiltering and washing the composition prepared according to thecopending application Serial No. 777,116, filed September 30, 1947, andnow Patent 2,592,870, finely divided copper, carbon black and colloidalgraphite. Examples of less volatile organic solvents which may bepresent in the slurries are the lower polyhydric alcohols, such asethylene glycol and glycerine and the aminoalcohols, such asmono-ethanolamine. The proportion by Weight of solids in the slurryvaries greatly depending upon the material. In the case of finelydivided silver the proportion may be about 4 to 1 Whilst with carbonblack 1 to 1 is suitable.

The organic electrically insulating material may be a thermoplastic orthermoset material or it may be a high polymer. It may be loaded withinorganic insulating materials such as asbestos, silica or glass fibres.Examples of suitable materials are electrical grades ofphenol-formaldehyde and aniline-formaldehyde synthetic resins,amino-plast resins such as urea-formaldehyde and melamine-formaldehyderesins, polyacrylic resins, such as polymethyl methacrylate, polyvinylresins, such as polystyrene and polythene. There may also be usedmixtures of thermoplastic and thermoset resins such as the so-calledstyrene copolymers and the products obtained by the use of such mixturesas phenol-formaldehyde condensation products with a minor proportion ofpolyvinyl chloride. These materials may be used in a variety of formssuch as sheet laminates, circular or elliptical laminates, arc-shapedlaminates, extruded forms, films and filled plates and blocks.

In describing one form of the invention it will be assumed that it isdesired to produce a printed circuit in silver upon aphenol-formaldehyde sheet material. The sheet material, which may firsthave had any holes that are desired bored in it, is substantiallyuniformly coated with finely divided silver. For this purpose a slurryof sponge silver in methanol may be sprayed on to the surface of thephenol-formaldehyde sheet. The greater part of the methanol is thenremoved by drying; a small amount is retained for temporarily adheringthe silver to the sheet. The sheet is then placed in a press and thecoated surface is brought into contact with a heated relief printingplate. This plate may be the usual type of relief copper printing plate.It is preferred to apply the maximum possible pressure and to use thelowest permissible temperature for this operation, which is of shortduration-generally of the order of 1l0 seconds. With aphenol-formaldehyde insulating material a pressure of about 1000 lbs./in. at about 200 C. and a contact time of 4 seconds has been foundsuitable.

When materials having a fairly well defined melting point are printedthe printing plate may be applied at a temperature somewhat above themelting point for a short interval of time. Thus polythene can beprinted using a plate maintained at a temperature some 20 C. above itsmelting point provided that the time of contact is sulficiently short e.g. 1 second. With thermoset materials quite high temperatures such as200-250 C. and high pressures such as 1000-2000 lbs/in. are preferred.Materials which soften on heating may be printed using a platemaintained at a much higher temperature than that at which they firstsoften. To some extent the time of contact appears to depend upon thearea being printed.

A continuous roll of material which has previously been coated withconductive material on one side may be printed by passing between tworollers one of which is heated and carries a relief print upon itssurface and is in contact with-the coated side of the material. Ifdesired the other side'of the material can be printed in a similarmanner in a subsequent step. This method is very suitable when shorttimes of contact are desired. Any desired pressure can be applied at thenip of the rollers.-

Where the silver or other conductor has not been in contact with therelief print, the conductor is, after the printing treatment,- veryloosely held and it can usually be brushed off or blown ofi bycompressed air; In this way it is usually possible to recover theunusedmaterial, especially in the case of a material like silver, whichis relatively expensive. I a

When a printed circuit has been produced in the manner described abovethe spaces that have been left for resistors can be overprinted. To dothis the whole of-the surface of the once printed plate may be coatedwith a poor conductor such as graphite inthe same way as alreadydescribed for silver and it is then again printed using a second reliefprinting plate which isprovided with relief portions corresponding onlyto the resistors required but allowing a slight overlap ontothe-conductors at the appropriate points in order to provide contacts.The unused graphite can then be brushed ofi. Alternatively, theresistors can be printed first and the metallic circuit printedthereafter. 7

Not only complete circuits but specific components such as coils can beprinted in the manner described.

An important feature'of-the process is that the conductor is applied tothe surface of the insulating material Without a permanent binder beingpresent. -A small amount of higher boiling organic liquid or the residueof the volatile diluent is relied upon to maintain a suflicientadherence of the particles of conductor to the insulating material andthis is volatilised in at least those areas to which the relief printingplate is applied. In the product the particles firmly 'cohere to'eachother and at least the lower portions thereof are firmly embedded in theinsulating material in the printed areas.

The higher boiling organic liquid may be so chosen as to'counteract anytendency of the conductor to undergo chemical change in the earlierstages of the process. Thus when sponge silver is used a small additionof an ethanolamine may be made to prevent oxidation to silver oxide orto reverse such change, if it has occurred, during the hot pressing.However since the reduction involved is'an exothermic process it is notpossible to use silver oxide per se in the coating step.

--The following examples illustrate the manner in which the inventionmay be carried into effect:

Example 1 A polythene sheet, having a melting point of about 115 C., of0.031 inch thickness was employed. It was sprayed with a slurry ofsponge silver in methanol containing 2% of glycerine- The slurrycontained 80 parts of silver for each ZO parts of diluent by weight. Airdrying was employed .to evaporate the greater part of the diluent. Thesubstantially dried sheet was transferred to a press and brought intocontact with a copper relief printing plate maintained at'approximately135 C., the pressure employed was 50 lbs./in. the time of contact wassomewhat less than one second, the sheet was removed from the press andthe unpressed silver blown off.

lf-the press is run at 120 C. a time of contact of 2-3 seconds isrequired. The surface of the sheet may be subsequently polished ifdesired.

Example 2 V A sheet of polymethyl-methacrylate, 0.25 inch thick andhaving an initial softening point of 80 C. was employed. 'It was sprayedwith carbon black of low oil absorption suspended in ethyl alcoholcontaining 2% 4 glycerine. in equal proportions by weight. The coatedsurface was substantially dried using infra-red heat and then placed ina press run at 145-160 C., the pressure employed was 500 lbs/infi timesof contact of from 3 to 6 seconds were used, the shorter times beingused at the higher temperatures. Carbon black was removed from theunpressed areas by brushing in water.

Example 3 A polystyrene film, 0.0005 inch thick was sprayed with spongesilver having an average particle size of 50 a as described inExample 1.It was passed between a pair of rollers adjusted to a pressure of 20lbs/in. at the nip. Temperatures between and C. were employed andtherate .of passage'was 4 feet per minute. After passing through therollers silver was removed from the unpressed areas by washing in atrough.

Example 4 An electrical grade phenol-formaldehyde laminateO. 125 inchthick was employed. Itwas sprayed with. asponge silver using methanolcontaining 5% .of ethanolamine by weight the slurry containing 80 partsof silver for each 20 parts of diluent by weight. The sprayed surfacewas substantially dried using infra-red heatand it was then transferredto a press whichwas run at 200? C. and a pressure of 1000 lbs/in Thetime of contact was 4 seconds. Unpressed silver was removed from thepress plate by brushing.

Example 5 The procedure of Example 4 was repeated using, in place ofsponge silver, electrolytic copper powder 400 mesh. 1

Example 6 This example illustrates the printing of resistors on to" abase already containing a printed circuit produced as in Example 4.

A slurry of colloidal graphite in water was evenly applied to the 'wholesurface of-the plate which had already been printedwith the silver partsof the circuit and it was-dried using infra-red heat at 140 C. The

coated plate was transferred to'a press; run at '175" C.

in Example 4. It was placed in the press and pressed for 4 seconds at185 .C. using 700 lbs./in. pressures. Resistances were printed on to'this plate in exactly the same manner as described in' Example 6.

Example 8.

This example illustrates the printing of a conducting circuit on to a.'base already containing resistors and is illustrated by the accompanyingdrawings. a

An electrical grade of phenol-formaldehyde laminate 15, 0.125 inch thickwas employed. A 'slurryof coI- loidal graphite vin water was evenlyapplied to the whole surface and it was dried using infra-red heatat 140C. The coated plate was transferred to a press run at C. and 1200lbs/in; and was incontact with the relief printing plate for .10seconds.- 'After removal from the press the plate was .washed well andbrushed in water. The.printed resistances 1, 2, 3, 4' are shown in Fig.1 of the drawings. 1 j

The plate was then printed with a silver circu'itin The carbon black andsolvent were employedthe manner outlined in Example 4, slight overlap onto the resistors being allowed at the appropriate points in order toensure contact. After printing and removing excess silver the producthas the appearance shown in Fig. 2 of the drawings. Numeral 5 indicatespoints for soldering tags. Numeral 6 shows connection points for asub-miniature valve holder; numerals 7 indicate soldering points forcomponents from the reverse side of the base, 8 is a seven-turn airwound coil, numerals 9 denote connection points for sub-miniature valveholders astride the chassis, numerals 10 indicate points for directsoldering connections, numeral 11 indicates a connection to the reverseside of the base by eyelet. Numerals 12 indicate contacts for plug inconnection to spring contacts while 13 is the soldering point for a foilconnection to a capacitor which has a printed base connection 14.

Fig. 3 of the drawings shows a crosssection taken along the line 111111of Fig. 2 and shows that the contacts 12 are partially embedded in thebase material.

What we claim is:

1. A process for imprinting a circuit element upon a substantiallysmooth surface of electrical insulating synthetic plastic material whichcomprises applying to the surface thereof, a slurry of finely dividedcircuit element forming particles in a volatile liquid that is inert tothe plastic, applying simultaneously to at least one selected area ofsaid slurry covered plastic and prior to completion of evaporation ofsaid volatile liquid therefrom, mechanical pressure and heat to causethe individual particles to cohere and to become at least partiallyembedded in the underlying insulating material without substantiallyimpairing the electrical properties of the surface of the selected areaof said material and thereafter removing electrical conductor from thoseareas of said covered plastic to which said pressure and heat have notbeen applied.

2. The process as set forth in claim 1 in which the circuit elementforming particles are of silver.

3. The process as set forth in claim 1 in which the circuit elementforming particles are of carbon black.

4. The process as set forth in claim 1 in which the volatile liquid isselected from the group consisting of water, the lower alkanols, thelower cyclic ethers, the lower aliphatic ketones, their derivatives andmixtures thereof.

5. The process as set forth in claim 4 in which the volatile liquidincludes a component of lower volatility to retain the solid particlesin place on the plastic material prior to application thereto of thebonding pressure and heat.

6. The process as set forth in claim 5 in which said component of lowervolatility is selected from the group consisting of lower polyhydricalcohols and aminoalcohols.

7. The process as set forth in claim 1 in which two sets of circuitforming components are successively applied to the plastic, one setbeing highly conductive components composed of metal particles, theother set being resistor elements composed of carbon black particles,and in which continuity of circuit between conductive and resistorelements is maintained by end overlapping the areas to which mechanicalpressure and heat are applied.

8. A process for imprinting a circuit element upon a substantiallysmooth surface of electrical insulating synthetic plastic material whichcomprises applying to the surface thereof a slurry of finely dividedcircuit element forming particles in a volatile liquid that is inert tothe plastic, applying simultaneously to at least one selected area ofthe slurry covered plastic and prior to completion of evaporation ofsaid volatile liquid therefrom, mechanical pressure of the order of 20to 2000 lbs. per square inch and heat of the order of to 250 C. for aperiod of time of up to 10 seconds with resultant complete evaporationof volatile liquid from said selected area, secure embedding of thecircuit element forming particles in the surface of the selected area ofsaid plastic material and coherence of such particles withoutsubstantially impairing the electrical properties of the surface of saidmaterial, and thereafter removing electrical conductor from those areasof said covered plastic to which said mechanical pressure and heat havenot been applied.

9. The process as set forth in claim 8 in which the circuit elementforming particles are of silver.

10. The process as set forth in claim 8 in which the circuit elementforming particles are of carbon black.

11. The process as set forth in claim 8 in which the volatile liquid isselected from the group consisting of water, the lower alkanols, thelower cyclic ethers, the lower aliphatic ketones, their derivatives andmixtures thereof.

12. The process as set forth in claim 11 in which the volatile liquidincludes a component of lower volatility to retain the solid particlesin place on the plastic material prior to application thereto of thebonding pressure and heat.

13. The process as set forth in claim 12 in which said component oflower volatility is selected from the group consisting of lowerpolyhydric alcohols and aminoalcohols.

14. The process as set forth in claim 8 in which two sets of circuitforming components are successively applied to the plastic, one setbeing highly conductive components composed of metal particles, theother set being resistor elements composed of carbon black particles,and in which continuity of circuit between conductive and resistorelements is maintained by end overlapping the areas to which mechanicalpressure and heat are applied.

References Cited in the file of this patent UNITED STATES PATENTS1,717,193 Dantsizen June 11, 1929 1,922,254 McCulloch Aug. 15, 19331,987,969 Parkin Jan. 15, 1935 2,121,005 Bener June 21, 1938 2,136,370Bockius et a1 Nov. 15, 1938 2,177,484 Fruth Oct. 24, 1939 2,191,556Carothers Feb. 27, 1940 2,429,089 Box Oct. 14, 1947 2,441,960 Eisler May25, 1948 2,473,183 Watson June 14, 1949 2,474,988 Sargrove July 5, 19492,492,429 John Dec. 27, 1949 FOREIGN PATENTS 269,729 Great Britain Apr.28, 1927 OTHER REFERENCES Printed Circuit, National Bureau of Standards,Circular 468, November 15, 1947, pp. 4, 6, 7, 12, 13, 14, 28.

1. A PROCESS FOR IMPRINTING A CIRCUIT ELEMENT UPON A SUBSTANTIALLYSMOOTH SURFACE OF ELECRICAL INSULATING SYNTHETIC PLASTIC MATERIAL WHICHCOMPRISES APPLYING TO THE SURFACE THEREOF, A SLURRY OF FINELY DIVIDEDCIRCUIT ELEMENT FORMING PARTICLES IN A VOLATILE LIQUID THAT IS INERT TOTHE PLASTIC, APPLYING SIMULTANEOUSLY TO AT LEAST ONE SELECTED AREA OFSAID SLURRY COVERED PLASTIC AND PRIOR TO COMPLETION OF EVAPORATION OFSAID VOLATILE LIQUID THEREFROM, MECHANICAL PRESSURE AND HEAT TO CAUSETHE INDIVIDUAL PARTICLES TO COHERE AND TO BECOME AT LEAST PARTILLYEMBEDDED IN THE UNDERLYING INSULATING MATERIAL WITHOUT SUBSTANTIALLYIMPAIRING THE ELECTRICAL PROPERTIES OF THE SURFACE OF THE SELECTED AREAOF SAID MATERIAL AND THEREAFTER REMOVING ELECTRICAL CONDUCTOR FROM THOSEAREAS OF SAID COVERED PLASTIC TO WHICH SAID PRESSURE AND HEAT HAVE NOTBEEN APPLIED.